28 A man, therefore, who acts in obedience to the faith which obeys God, restrains all mortal affections, and keeps them within the natural limit, regulating his desires so as to put the higher before the lower. If there was no pleasure in what is unlawful, no one would sin. To sin is to indulge this pleasure instead of restraining it. And by unlawful is meant what is forbidden by the law in which the order of nature is preserved. It is a great question whether there is any rational creature for which there is no pleasure in what is unlawful.
If there is such a class of creatures, it does not include man, nor that angelic nature which abode not in the truth. These rational creatures were so made, that they had the potentiality of restraining their desires from the unlawful; and in not doing this they sinned. Great, then, is the creature man, for he is restored by this potentiality, by which, if he had so chosen, he would not have fallen. And great is the Lord, and greatly to be praised, who created man. For He created also inferior natures which cannot sin, and superior natures which will not sin.
Beasts do not sin, for their nature agrees with the eternal law from being subject to it, without being in possession of it. And again, angels do not sin, because their heavenly nature is so in possession of the eternal law that God is the only object of its desire, and they obey His will without any experience of temptation. But man, whose life on this earth is a trial on account of sin, subdues to himself what he has in common with beasts, and subdues to God what he has in common with angels; till, when righteousness is perfected and immortality attained, he shall be raised from among beasts and ranked with angels.
Source: Reply to Faustus the Manichaean (New Advent)